Fastening of moving blades in turbine rotors by anchoring blade roots in a circumferential slot is known. Such fastening is used in particular in the case of smaller blades. The blade roots, in their horizontal cross section perpendicular to the blade longitudinal axis, usually have a basic rhombic shape and, in their longitudinal axis, basically have the shape of an inverted T or a hammer-head shape, which engages in serrations in the slot. In this case, the short sides, of a rhombus or axial guide surfaces, are bearing against the side walls of the slot.
During the fitting of blades in circumferential slots in the rotor, the problem of fitting the last blade of a blade row arises in each case. A known method is to ensure that there is a fitting gap, which is dimensioned in such a way that the last blade root can only just be inserted into the slot. If the blade root has a basic rhombic shape, the fitting gap needs to be at least equal to the long diagonal of the rhombus, as shown in FIG. 1a and explained in more detail below. After the last blade has been inserted and rotated into the desired orientation, a residual gap remains between the first blade and the last (nth) blade. To fix the blades in the slot and to avoid displacements of the blades during operation, this residual gap is filled by means of intermediate pieces or distancing inserts which are arranged between the blade roots, as shown in FIGS. 1b and c and explained in more detail below. FIGS. 2a and b show individual intermediate pieces, some of which are whole intermediate pieces and others are halved intermediate pieces. In the case of the halved intermediate pieces, the sides bearing against one another in the center are straight and vertical. The intermediate pieces or distancing inserts serve not only to fix the blade roots in the slot but also to keep the blade roots at a distance from one another and to uniformly distribute the residual gap over the circumference of the slot.
The intermediate pieces in each case have the same shape as the blade roots which fix them. The blade roots of each blade row are in each case dimensioned in accordance with the size of the blades of that blade row. In this case, the blade roots of one blade row differ from those of a further blade row in their width, their length and the number of serrations and steps in the T-shape or hammer-head shape.
During the fitting of the last intermediate piece, the same problem as with the fitting of the last blade again appears. To solve this problem, the presence of a gap which is large enough for fitting a halved intermediate piece is again ensured. Finally, this smaller gap is filled by means of halved intermediate pieces, as shown in FIG. 2b. (the gap which still remains at the end after insertion of these halved intermediate pieces is finally filled by a three-piece closing piece, whereupon the blades are sufficiently fixed in the circumferential slot for the operation of the turbine.)
The basic shape, or the cross section, of the whole intermediate pieces described here is rectangular, i.e. the short sides of the intermediate piece, or axial guide surfaces, lie at an angle to the side wall of the circumferential slot. The intermediate pieces therefore touch the side wall of the slot only with their corners. However, the slot and the intermediate pieces are not greatly loaded as a result, since the lateral axial force on the whole intermediate pieces is only very small.
In the fitting region, the halved intermediate pieces have a rhombic cross section, the short sides of the rhombus being at an angle to the long sides which is different from 90.degree.. The angle is dimensioned in such a way that the short sides, or axial guide surfaces, bear flat against the side wall of the circumferential slot. Flat bearing is necessary since the lateral axial forces which act on the halved intermediate piece are much greater compared with the whole intermediate pieces. If the short sides did not bear flat against the side walls of the slot, the halved intermediate pieces would cut a notch in the side wall and damage the latter.
The difference between the axial forces on the halved intermediate pieces and the axial forces on the whole intermediate pieces is connected with the fact that there is one center of gravity in the case of the whole intermediate pieces and there are two centers of gravity, on which the centrifugal force acts, in the case of the halved intermediate pieces.
The angled configuration of the short sides of the halved intermediate pieces avoids damage to the intermediate pieces and the slot by notching. However, the production of the angled configuration is expensive.